Introduction to t he t reatment of tannery effluents
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INTRODUCTION TO T HE T REATMENT OF TANNERY EFFLUENTS. What every tanner should know about effluent treatment. U nited N ations I ndustrial D evelopment O rganization. Part IV. Compiled by J . Buljan , I. Kral. 4. Biological (secondary) treatment.

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Introduction to t he t reatment of tannery effluents

INTRODUCTIONTOTHE TREATMENTOF TANNERY EFFLUENTS

What every tanner should knowabout effluent treatment

United NationsIndustrialDevelopmentOrganization

Part IV

Compiled byJ. Buljan, I. Kral


4 biological secondary treatment
4. Biological (secondary) treatment

  • Activated sludge process – principle, parameters

  • Aeration devices

  • Oxidation ditch

  • Flow-chart of the biological (secondary) treatment

  • Flow-chart of the fully-fledged treatment


Activated sludge process
Activated sludge process

Air

CLARIFIER

Effluent

Influent

Sludge return

AERATION TANK

Q+R

Sludge

recirculation

pump

Q

R

Excess of sludge


Activated sludge process1
Activated sludge process

  • Total Influent Volume, Q:Volume of the treated effluent [m3/day]

  • Tank Volume, V:Aeration tank volume [m3]

  • Organic Loading, F: The total BOD5 applied [kg/day]

    F = (f * Q)/1000

    where f – the BOD5 of the influent [mg/l]

  • Mixed Liquor Suspended Solids, M:

    M = (MLSS * V)/1000

    where MLSS - is the concentration of SS in Mixed Liquor in the aeration tank [mg/l]

  • Loading factor, F/M: BOD5 kg per day per kg of Mixed Liquor Suspended

    Solids (MLSS) in the aeration tank [mg/l].

  • Hydraulic retention time, T:

    T = (V/Q) * 24

    where V – aeration tank volume [m3]



Other operational parameters
Other operational parameters ratios

  • Dissolved Oxygen, DO:

    The content of molecular oxygen in the aeration tank [mg O2/l].

    Usually DO = 0.5-2.0 mg O2/l.

  • pH:

    The optimum pH range for aerobic process is 7.0-7.5

  • Temperature:

    Affects the metabolic and growth rates of the organisms.

    Optimum: 10-30oC.

  • Nutrients, desirable ratio:

    BOD : N : P = 100 : 5 : 1

  • Sludge Volume Index, SVI:

    Volume of 1 g of activated sludge after settling the aerated mixed liquor in a 1000 ml graduated cylinder or Imhoff cone for 30 min.

    Desired value < 100 ml/g.


Aeration devices
Aeration devices ratios

There are many types and subtypes of aerators, models being continuously improved. Here is an arbitrary classification of types frequently installed in tannery ETPs.

  • Surface aerators

    • Radial flow, low speed, 20-60 rpm

    • Axial flow, high speed, 300-1200 rpm

    • Brush rotor (oxidation ditch) 

  • Submerged turbines 

  • Diffused air 

    • Bubblers – porous and nonporous diffusers

    • Tubular

    • Jets (developed from Venturi ejectors)


Classification of mechanical aerators
Classification ratiosof mechanical aerators










Submersed turbine aerators in the oxidation ditch
Submersed turbine aerators ratiosin the oxidation ditch



Purification efficiency of the treatment stages referred to raw effluent
Purification efficiency of the treatment stages referred to raw effluent

* Approximately at the load level of 0.30 kg BOD5/kg MLSS, oxygen requirement of

2.0 kg O2/kg BOD5 and MLSS 3300 g/m3.


Biological treatment stage simplified flow chart
Biological treatment stage raw effluentSimplified flow-chart

AIR

Submersible pump

AIR

AIR

Pump

PRIMARY TREATED

EFFLUENT

Motor

BLOWER

Gate valve

TREATED

EFFLUENT

FOR DISCHARGE

AERATION TANK 1

AERATION TANK 2

(DIFFUSED AERATION SYSTEM)

END FLOW MEASURING

WITH FLOW SUMMARISING

SECONDARY SETTLING

TANK

FLOW MEASUREMENT

CHANNEL

SLUDGE

THICKENER

G

CHAMBER

FILTER PRESS

SECONDARY SLUDGE

PUMPING STATION

SLUDGE DRYING

BEDS (optional)


Fully fledged treatment system

Submersible pump raw effluent

Dosing pump

SERVICE WATER

Pump

POLYELECTROLYTE

LIME

ALUM

CATALYST

MnSO4

Motor

Ball valve

AIR

Gate valve

Agitator

Drain

Drain

Drain

Drain

MIXED EFFLUENT

BLOWER

Fully-fledged treatment system

AUTOMATIC

COARSE SCREEN

FINE SCREEN

INLET PUMPING STATION

EQUALIZATION TANKWITH PUMPING STATION

(WITH EJECTORS FOR MIXING AND AERATION)

AERATION TANK 1

AERATION TANK 2

(DIFFUSED AERATION SYSTEM)

SECONDARY

SETTLING TANK

AIR

AIR

PRIMARY

SETTLING TANK

END FLOW MEASURING

WITH FLOW SUMMARISING

TREATED

EFFLUENT

FOR DISCHARGE

FLOCCULATION

TANK

COAGULATION

TANK

FLOW MEASUREMENT

CHANNEL

G

SLUDGE DRYING

BEDS (optional)

CHAMBER

FILTER PRESS

SECONDARY SLUDGE

PUMPING STATION

SLUDGE

THICKENER

PRIMARY SLUDGE

PUMPING STATION


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